[b]Blacklight Retribution[/b] should get down mileage for free. Same goes for [b]Dirty Bomb[/b]. [b]HAWKEN[/b] too. [b]Risk of Rain[/b] should be cheap enough for the group, but whether they like a side-scrolling roguelike or not is the question. Same kinda goes for [b]Dungeon of the Endless[/b], but with less side-scrolling and more tower defense and hero management. The [b]Sanctum[/b] games might be of some use if you enjoy shooters and tower defense games. [b]E.Y.E. Divine Cybermancy[/b] is going to be odd yet probably interesting. Cheap too. [b]Trackmania[/b] games are always a fun arcade racer. They even have a free version that some argue is better than most of the paid versions. Quite a few of the other games mentioned already have my recommendation.